Various methods and devices have been proposed for inspecting containers for purposes of identifying contraband and other potentially harmful materials which may be used for terrorism or for other unlawful activities. Massive amounts of cargo are unloaded, and thereafter inspected for Customs or other regulatory purposes, at ports of entry to the United States. This inspection process is not without its shortcomings. It is well known that contraband has often slipped past inspectors and other government agents by being positioned or otherwise concealed within large, sealed shipping containers used to ship so-called “containerized freight,” which can be offloaded onto tractor trailers, rendering detection of the contraband difficult, if not impossible, to uncover using conventional means. So-called “palletized” freight, wherein a large number of boxes or other objects are secured to transport pallets by strapping and shrink-wrapping with heavy plastic, present similar detection challenges. In addition to the foregoing, nuclear materials, even relatively small quantities of which could be effectively utilized in an explosive device in the form of a thermonuclear explosive or in a so-called “dirty bomb” wherein radioactive material is widely dispersed using conventional explosives, may be radiation-shielding and enclosed in a relatively small region of a large shipping container. Effective detection and identification of contraband, including concealed, shielded high-density nuclear material, therefore, is a priority at United States ports of entry. Similarly, effective airport security has become of grave concern, given the ease with which explosives may be hidden in both checked and carry-on baggage.
Systems for conducting detection of contraband, including both conventional explosives and nuclear materials, are known in the art. A number of these systems use directed beams of photons, which may also be characterized in non-technical terms as “X-rays,” to generate a detector response to the presence of such undesirable materials, including radiation-shielded nuclear materials. See, for example, U.S. Pat. Nos. 5,115,459 and 5,838,759, and U.S. Patent Publications US2005/0117683, US2006/0140341, and US2007/0245809, the disclosure of each of which document is incorporated herein in its entirety by this reference. See also, for example, the following publications of the Idaho National Laboratory (INL) (formerly the Idaho National Engineering and Environmental Laboratory (INEEL)): “Proof-of-Concept Assessment of a Photofission-Based Interrogation System for the Detection of Shielded Nuclear Material,” INEEL/EXT-2000-01523, November 2000; “Pulsed Photonuclear Assessment (PPA) Technique: CY-05 Project Summary Report,” INL/EXT-05-01020, December 2005; and Pulsed Photonuclear Assessment (PPA) Technology Enhancement Study, INL/EXT-06-11175, April 2006, the disclosure of each of which document is incorporated herein in its entirety by this reference. See, also, “PITAS Generation III System Design Report The Developmental Prototype,” INL/EXT-08-13798, January 2008.
A significant disadvantage of conventional systems which may be used to detect nuclear material, including shielded nuclear material, is their inability to handle the sheer volume of cargo entering the United States. In particular, scanning freight as it is offloaded from transport vessels and prior to disposition on trucks for domestic transport is an overwhelming task, given the millions of units of containerized and palletized freight offloaded at U.S. ports each year. Given the objective of scanning all incoming foreign freight, even conducting a container-by-container scan is impractical from both cost and time standpoints. In addition, conducting the inspection process after the freight has reached port and in the presence of a large number of personnel presents small, but notable risks to property and human life.
As a result, it would be desirable to develop a detection system with the capability of scanning cargo vessels (such term including sea, air and land transport vessels) at a rapid rate and at considerable standoff distances. However, conventional photonuclear-based detection technology is unsuitable for detection at distances in excess of a few meters.
For example, it has been recognized by the inventors herein that the use of linacs to generate a photon beam using an electron source of relatively high energy, for example, and not by way of limitation, in the range of about 8 MeV to about 100 MeV in high-standoff field operations such as the aforementioned cargo vessel scanning applications, requires that off-axis (e.g., diverging from the main radiation beam path) radiation doses be minimized. In addition to controlling off-axis radiation, the inventors have recognized that it is desirable to have substantially only high-energy photons on the beam axis, to limit the radiation dose while maximizing photonuclear stimulation of a shielded nuclear target material at substantial standoff distances.
While near-field use of a photon beam to generate a response from a target material can be effective inside a shielded cell with adequate off-axis dose controls, field operations with much larger standoff distances, on the order of hundreds of meters, are not susceptible to the use of such traditional methods.
Further, a major contributor in the use of conventional linac systems to an on-axis radiation dose to a targeted inspection area is from low energy photons from the bremsstrahlung process used to generate the photon beam, and such low energy photons do not provide the desired photonuclear signature from the target material.